Carrier posibly an impact?
I have a similar setup. However instead of the Draco Design Supernova, I have the cheap (~$12) Love Mei knock off called the "Slim Waist Bumper".
Finish is not as good as the Draco (all the milling marks are still evident, rather than the quality finish of the Draco, so you get what you pay for. A cheap prices for a cheap case).
Note, I'm not sure but possibly the Wireless Charging Back is helping? Besides the charging pad, there's an extended antenna for the NFC (normally from the battery). Perhaps it's used for more than NFC and thus negates some of the metal case impact?
I do see from the FCC images for T-Mobile, Verizon and Sprint that antenna layouts differ between those devices. Perhaps this impacts the signal as well.
That said, I found that I still have extremely close to the same amount of phone signal, no appreciable Wi-Fi impact (I use 802.11ac, but it's been solid everywhere I use wireless) and the GPS shows accurate to 16 feet when using the GPS testing software. Removing the case yields a stronger GPS, but the accuracy seems to be similar (14 feet). So perhaps it's where I'm located and the number of SATs in view (15).
Downtown Seattle, GPS is pretty spotty but I suspect the skyscrapers blocking line of site is the reason. But in urban and suburban locations it's pretty solid. I've used the Google Navigation functions multiple times with the phone and case and have arrived to my destination every time.
The use of a glass screen protector, this case and the official Wireless Charging Back have provided me a nice looking phone.